Manhattan Project II. The Race for the Bomb The theoretical possibility that an explosion could be brought about by atomic fission became known in 1939, the year that war broke out in Europe. Scientists discovered then that uranium atoms can fission when struck by neutrons to split other atoms in a chain reaction, releasing large amounts of energy. Two Hungarian physicists who had recently emigrated to the United States, Leo Szilard and Eugene Wigner, alerted the US government to the possibility of an atomic bomb. Along with Albert Einstein, they wrote a letter to President Roosevelt warning that Nazi Germany might also be working towards a uranium bomb; many of the important discoveries in atomic physics had been made at German universities.

Roosevelt responded by setting up an advisory committee on uranium in October 1939. Under the aegis of this committee, American scientists at several centres examined the problem. The uranium that fissions is an isotope, a variation of an element that is chemically indistinguishable but different in its atomic structure. It is uranium-235, which constitutes only 0. 7 per cent of uranium. Scientists questioned whether significant quantities could ever be separated.

Much of the initial work was done at Columbia University in New York, and the military direction was from an office in Manhattan. This was located in the Manhattan Engineering District, and the whole programme became the Manhattan Project, under the command of Major General Leslie Groves. Meanwhile, in Britain, two immigrant physicists, the Austrian Otto Frisch and the German Rudolf Peierls, decided that an atomic explosion using uranium-235 was a practical possibility. They alerted the British authorities in early 1940 with a memorandum showing how such a bomb could be produced.

After more studies, the British government set up a project to build an atomic bomb. The United States and Britain exchanged information on weapons developments. British progress towards an atomic bomb convinced American scientists in the summer of 1941 that it could be done, and spurred on their work. In November 1941 a committee of scientists recommended that the United States embark on an all-out research programme, and President Roosevelt gave the go-ahead the following January. In the intervening weeks the Japanese attack on Pearl Harbor had brought the United States into the war, and provided an added emergency.

American scientists were still worried that Germany might be working on an atomic bomb. Scientific work was concentrated at first at the University of Chicago; scientists there working under the Italian Enrico Fermi created the world's first atomic chain reaction in an atomic reactor in December 1942.